1. Field of the Invention
This invention relates to the field of injection molding. More particularly, the invention relates to a tip insulator assembly that attaches to a distal end of an injection nozzle. The tip insulator assembly is held onto the nozzle preferably by a removable retainer means.
2. Summary of the Prior Art
Insulators are used to thermally insulate the heated tip or nozzle of a hot runner nozzle assembly from the surrounding cooled gate insert of a mold cavity. U.S. Pat. No. 4,662,837 to Anderson (incorporated herein by reference) shows such an insulator assembly. The insulative material is typically made of a high temperature resistant, resilient material such as VESPEL (polyimide resin). The insulators occupy space that would otherwise be filled with a resin xe2x80x9cbubblexe2x80x9d, well known in the art. If the resin being processed tends to degrade over time and it is allowed to form the insulating bubble around the tip, the degraded resin eventually is drawn into each part being molded causing unsatisfactory properties and appearance. It is therefore essential that the insulating function of the bubble be performed by some other material other than the resin being processed if it is thermally sensitive or if plastics of differing colors are to be processed and errant color streaks are undesirable.
U.S. Pat. No. 5,208,052 to Schmidt et al. (incorporated herein by reference) shows another insulator construction that surrounds a heated tip of a hot runner nozzle assembly. The insulator is made of titanium and separated from both the tip and cooled gate area by air gaps on either side. The insulator is threaded onto the nozzle housing and retains the tip in the housing by means of the contact area there between. The tip must seal against the bubble wall to prevent resin leaking behind it and occupying the air gap space. Sealing is effected by a seal in combination with deflection of the insulator leg pressing between the tip and the bubble wall. This design has the disadvantage of being relatively costly and titanium is not as effective as a thermal insulator as VESPEL (polyimide resin).
Co-pending U.S. application Ser. No. 09/272,251, filed Mar. 19, 1999, now U.S. Pat. No. 6,315,549, to Jenko et. al, shows a two piece insulator that surrounds the tip. An inner titanium sleeve is surrounded by a rather large outer VESPEL (polyimide resin) sleeve and the assembly is releasably fastened to an injection nozzle. In field testing has revealed that the structural design of the Jenko titanium inner sleeve is less than adequate if large enough preload forces are applied to the nozzle tip. Due to the location and orientation of the shoulder on the inner sleeve of the Jenko insulator assembly, a significant amount of the preload is applied to the unsupported shoulder which results in tensile failure of the inner sleeve.
The primary objective of the present invention is to provide a nozzle tip insulator assembly that may be releasably connected to an injection nozzle and withstand the pressure and temperature associated with injection molding plastics thereby overcoming the drawbacks of the prior art.
Another object of the present invention is to provide a nozzle tip insulator assembly that is comprised of a low profile xe2x80x9cVespelxe2x80x9d outer insulator sleeve that is positively retained to an inner titanium sleeve when the mold plates are opened.
Another object of the present invention is to provide a nozzle tip insulator assembly that is comprised of an outer xe2x80x9cVespelxe2x80x9d insulator sleeve that is retained in place on an inner titanium sleeve during an injection molding cycle or cycles.
Another object of the present invention is to provide a nozzle tip insulator assembly that uses a smaller amount of expensive xe2x80x9cVespelxe2x80x9d to reduce cost and to further reduce the amount of heat transferred out of the nozzle.
Another object of the present invention is to provide a nozzle tip insulator assembly that provides improved structural performance over the prior art.
Another object of the invention is to provide a nozzle tip insulator that acts as the only nozzle seal for an injection nozzle.
A nozzle tip insulator assembly comprises an inner sleeve formed to fit around a nozzle tip, an outer sleeve on the inner sleeve, and a joining means located between the inner and outer sleeves. A shoulder protrudes from the inner sleeve to restrain the movement of the outer sleeve. The shoulder is located to direct a portion of loads applied to the outer sleeve into the nozzle tip, thereby reducing tensile loads in the inner sleeve. The inner sleeve is preferably releasably affixed to an injection nozzle. The outer sleeve is preferably made of a material with a low thermal conductivity, such as VESPEL (polyimide resin).
Further objects and advantages of the present invention will appear hereinbelow.